Outgoing emails bounce back with error 452 too many recipients in last hour

[#1]

Most outgoing messages from all of our accounts immediately bounce back with the following message from the System Administrator:
Your message did not reach some or all of the intended recipients.
      Subject: check
      Sent: 25/10/2014 15:08
The following recipient(s) cannot be reached:
    'Email address' on 25/10/2014 15:08
            452 <email address> too many recipients in last hour
It has been happening for over 36 hours; note that  we do not use our account for sending out bulk emails; Incoming mail not affected but virtually every outgoing email bounces back.
I was told when I raised a ticket with Plusnet Support that it could be "tarpitting" ...caused by what ... and how do I fix this?
I have freshly scanned each PC but AVG cannot find any threats/viruses.
Could be "something" doesn't really help ... expect more support from Plusnet to remedy this situation. I have a business to run!
We use Outlook 2007 with POP3 accounts and not MS Exchange Server.
Has anyone else had this and know how I fix this please?

[#2]

Are all of these emails sent from the same domain name and if so, is it the .co.uk domain hosted against your Plusnet account?
Assuming you're using your Plusnet connection, the hourly recipient limit is typically around 1000 addresses. You're not sending more mail than this presumably?
Do you have any devices off of your network that might be configured to send using SMTP authentication and your Plusnet account details?